The present invention generally relates to desulfurization of flue gases of a steam generator, such as in a caloric power plant, as well as to the production of construction material formed of synthetic gypsum. More particularly, the present invention relates to a method for desulfurizing the ash-containing exhaust gas of the flue gases, especially in connection with the application or burning of brown coal, where the furnace is charged with powdered lime that is split into CaO and CO.sub.2, the CaO being transformed into CaSO.sub.3 or CaSO.sub.4 by absorption of SO.sub.2 and an excess of air (O.sub.2), at least on the surface of the powder particles thereof, and the powder particles being separated from the exhaust gas along with the ash particles. The present invention is also directed to an apparatus for pulverizing and separating ash-containing particulate matter.
A method for desulfurizing flue gas is already known in which the furnace is charged with limestone which reacts with the SO.sub.2 within the flue gas after splitting into CaO. During this method, the charged limestone (CaCO.sub.3) is split into calcium oxide (CaO) and carbon dioxide (CO.sub.2), with the calcium oxide (CaO) reacting with the sulfur dioxide (SO.sub.2) of the flue gas and one-half mole of oxygen (O.sub.2) to form calcium sulfate (CaSO.sub.4). However, this method is disadvantageous in that only one-tenth of the charged calcium is utilized, while the remaining 90% of the calcium is precipitated with the ash and the formed calcium sulfate.
Extension of contact time between flue gas and the limestone additive has been attempted by extending the time of direct contact of the particulate or dust contained in the dust filter with the flue gas, in order to achieve more efficient utilization of the limestone additive. However, calcium oxide (CaO) is very inactive within the normal operating temperature range of the filter, which is about 120.degree. to 160.degree. C. Appreciable improvement in utilization of the limestone additive can still be attained by cooling the flue gas to a temperature between 50.degree. and 100.degree. C., and by additionally increasing the relative humidity.
German Patent DE-OS No. 29 34 109 discloses that wet and dry slag and ash resulting from the furnace may be pulverized and, together with the absorbent, added to exhaust gas at various locations. This is disadvantageous in that milling of slag and ash can consume a great deal of energy, resulting in uneconomical production.
DE-OS No. 29 10 537, discloses that absorbents can be pulverized in a flow of steam and again added directly to the exhaust gas without valence saturation. This method results in a slightly better utilization of the absorbent, since only valences exposed at the broken or fractured surfaces, and ideally a molecular layer, are available for bonding of a gas molecule. The reaction product formed at the surfaces thereof immediately forms impermeable layer, even on a reactive sorbent such as calcium hydroxide (Ca(OH).sub.2). With limestone ash as the additive, the effect of simple pulverization in a jet mill and immediate charging into the flue gas current is particularly low, since a transformation of calcium oxide into calcium hydroxide with the steam of the flue gas would additionally have to occur. This transformation can take place only in the absence of carbon dioxide and sulfur dioxide, in other words in a pure water vapor atmosphere and with adequate contact time. Another known process is where the dust-like sorbent, instead of being fed directly into the furnace, is initially fed into the utilized gas stream and is cooled down to 120.degree. to 160.degree. C., in order to improve the efficiency of the solid sorbent. The sorbent is brought into contact with the flue gas several times in this particular method, with the sorbent being precipitated out over and over again. This particular process can be further improved upon, if the precipitated sorbent is moistened prior to being combined with the flue gas. The humidity content of the sorbent is simultaneously increased to 10%. Moistening can be carried out by way of wet steam in a continuous conveyor furnace where condensation takes place, or by treating the powderized lime sorbent with wet air. Utilization with slaked lime sorbent illustrates that in a contact path developed as a venturi path, the utilization of slaked lime as well as the overall degree of the sulfurization could be improved if the sorbent is slightly moistened.
This known process is not suitable for application of lime ash additive, since the remaining calcium oxide is virtually inactive at the particular temperature of 120.degree. to 160.degree. C. Treating the ash with steam as has been suggested may be adequate to stimulate the bonding of calcium hydroxide and sulfur dioxide, but it is inadequate for transforming the calcium oxide into the calcium hydroxide which is an indispensible intermediate reaction step for the subsequent bonding of the sulfur dioxide. While slaked lime is an extremely active, although very costly, absorbent, cooling of the flue gas below 100.degree. C. means that the flue gas might ultimately have to be reheated before exiting from the flow system, e.g. flowing out of a chimney, if it is required to satisfy local conditions. Humidity treatment of sorbents and possibly of ash can result in considerable stoppages, for even a small excess of water or minor inhomogeneity can cause sticking and caking of the dust-like sorbent.